Nitrogen Complexes of the Platinum Metals POINTERS TO A MECHANISM OF FIXATION By Professor Joseph Chatt, F.R.S. University of Sussex molecule was activated directly by a molyb- The discovery three years ago of thefirst denum ion in nitrogenase, the nitrogen fixing well-defined complex compound con- enzyme. This led to attempts, using nitrogen taining the nitrogen molecule - a under pressure, to obtain complexes of complex of ruthenium - aroused great molybdenum containing the nitrogen mole- interest in such compounds and in their cule, but of no avail, and the work went relation to nitrogen fixation. Since largely unpublished. then a number of other nitrogen com- The formal analogy between the triple plexes have been obtained, but as yet, bond in acetylene and nitrogen molecules despite reports to the contrary, attempts also led chemists to attempt the preparation of to reduce their nitrogen to ammonia nitrogen complexes of the platinum metals, have been unsuccessful. In this article analogous to the acetylene and olefin com- the author, who is Director of the plexes. It was again of no avail. We now know Agricultural Research Council's Unit that the electronic energy levels in the nitro- of Nitrogen Fixation, puts into perspec- gen and acetylene molecules are completely tive the considerable amount of work different. Thus the highest filled electronic now being done in thisfield, which may level in nitrogen is a o-molecular orbital at eventually point the way to a mechan- -15.6 electron volts, whereas in acetylene it ism by which atmospheric nitrogen may is a much more accessible degenerate pair of be fixed and then reduced to yield n-molecular orbitalsat -11.4electronvolts (2). ammonia and, for example, amines for Chemists were beginning to accept the view the plastics industry. that nitrogen did not form stable complex compounds, when by chance, just three years Since the 1930s chemists have thought that ago, Allen and Senoff in Toronto obtained co-ordination compounds containing the indirectly the first nitrogen complex, nitrogen molecule as a ligand might exist. [Ru(NH,),(N,)]~'-(3). Its discovery revived They were stimulated into this line of world-wide interest in nitrogen complexes thought by the discovery, in 1930, that generally and in their relation to nitrogen nitrogen-fixing bacteria metabolised nitrogen fixation by bacteria. Since then experiments gas only in the presence of traces of molyb- in widely separated laboratories in various denum. Nevertheless, the bacteria would parts of the world have thrown up some dozen grow without molybdenum if they were nitrogen complexes. These vary in stability provided with a source of fixed nitrogen such from the thermally stable and chemically as urea or ammonia (I). Molybdenum is inert osmium complex [OS(NH,),(N,)]~+ to evidently involved in the nitrogen fixing the highly reactive and thermally labile un- process, and it was thought that the nitrogen charged complexes of iron, cobalt and nickel. Platinum MetalsTev., 1969, 13, (l), 9-14 9 Despite reports to the contrary, the nitrogen and is converted to [RU(NH~)~(N,)]~+by ligand has not been reduced to ammonia in ammonia (8, 9). any complex (4). The product of reaction 2 is grossly con- So far only the Group VIII metals are taminated with a hydrazine complex, perhaps known to form nitrogen complexes, and those [RU(NHJ~(N,H~)]~+,which cannot be re- of cobalt, ruthenium and iridium have received moved by re-crystallisation, but it can be most study. This article is not concerned with destroyed by mercuric chloride oxidation nitrogen complexes of the first transition leaving the relatively inert nitrogen complex series. They are difficult to obtain pure, owing untouched. This impurity was not suspected to their extreme lability and sensitivity to until recently and many of the chemical oxidation, but the cobalt compound formu- properties recorded as belonging to the lated, perhaps incorrectly, as [Co(N,)(PPh,),] ruthenium nitrogen complex are those of a is important. It was the first nitrogen complex hydrazine complex. The most important reported to be obtained rapidly in high yield of these reactions was the supposed reduction from gaseous nitrogen at atmospheric pressure of the nitrogen ligand to ammonia by sodium (5). The hydride [CoH(N,)(PPh,),] is equally borohydride. However, tracer studies using important because it was the first nitrogen [Ru(NH ,) 5( 16N,)I 2+ show that no 15NH complex to be obtained in a protic solvent. It is obtained by sodium borohydride, or any is obtained by passing nitrogen gas through a other tried reductant (4, 9). The ammonia solution of a cobalt hydride complex in previously reported as coming from the ethanol according to the following reversible nitrogen ligand in this complex must have reaction (6). There is some doubt as to had its origin in the hydrazine. whether the two cobalt nitrogen The formation of the nitrogen complex by Harrison and Taube's method (Reactions [CoH3(PPh3),l +N& [CoH(N,)(PPh,),] +H, r 4 and 5) is important because it shows that complexes are different, but our experience nitrogen gas can compete successfully with suggests that both exist, although the non- hydridic complex is usually contaminated Table 1 with not less than 15 per cent of the hydride. Nitrogen Complexes of Platinum Metals The best-defined complexes of the platinum metals are listed in Table I. The most import- Ir Ref. ant are the ammine complexes of ruthenium, (X=CI, Br, N3) 22105 3 (X=CI, Br, N3) obtained in good yields by the following 7 3) reactions in aqueous solution. In reaction 3 (X=CI, Br, 21525 I9 the co-ordinated azide ion decomposes to Ru leave nitrogen on the metal. trans-[IrX(N,)(PPh,),] 21525 I1 2152521525 I2 RuC13 +N,H,-~[Ru(NH~),(N,)IC~Z 2 (3) Os(NH3)KFJL [Ru(NH3)5N312+a-t[Ru(NH3),(N,)lz' 3 (3) Ir (X=CI, Br, N3) 2[Ru(NH3)6Hz0I2 'SNa-4 221525 I4 [VWNH3) 5)2(N 211 *+ + H 20 4 (7) Ir [{Ru(NH,),)z(Nz)I4++NH3--> trans-[IrX(N,)(PPh,),] 2095 I3 [Ru(NH3),(Nz)12'+[Ru(NHJJ2'~ 5 (7) (X=CI, Br, N3) (see text) Ruthenium trichloride also picks up nitrogen A few other poorly defined complexes have to give a material of analysis, RuCl,(N,) been claimed, e.g. see ref. 20. No complex of Pd or Pt is known. (H,O),(THF), when its tetrahydrofuran t of chloride, solid state spectra. (THF) solution is reduced by zinc amalgam t [BPh,]- salt. under nitrogen. It has v(NrN) at z153cm-' Platinum MetalsTev., 1969, 13, (1) 10 water for a suitable metal site in aqueous The complex reacts with x-bonding ligands, solution. Reaction 4 even occurs when air is particularly acetylenes, to lose nitrogen. The bubbled through the aqua-ruthenium com- corresponding rhodium complex is formed by plex solution, but oxidation of the ruthenium an analogous reaction and is very unstable complex also occurs (10). It is the only known (14). reaction of nitrogen at ordinary temperature In addition to the reasonably-defined and pressure in aqueous solution, and might nitrogen complexes listed in the table and well provide a model for the uptake of nitro- discussed above, a number of other nitrogen gen, possibly by iron, in the enzyme nitro- complexes too unstable to be identified have genase. However, it does not lead to an easily been reported but none containing platinum reducible species, and if it provides a true or palladium. model the reduction step still has to be worked In the above mono-nuclear complexes the out. nitrogen is always bonded end-on to the The osmium complexes are remarkably metal as is carbon monoxide in the carbonyl stable. The analogue of the ruthenium complexes. The mode of bonding in nitrogen complex, [OS(NH,),(N,)]~+is obtained by complexes is also very similar to that in reaction of hydrazine on aqueous ammonium carbonyl complexes with back donation of chloro-osmate(1V) (II). The nitrogen is so non-bonding d-electrons from the metal into strongly bound that it is not lost even in the anti-bonding orbitals of the nitrogen boiling hydrochloric acid but it yields to a molecule as shown in the diagram. The mixture of hydroiodic acid and iodine to form feedback of electrons into anti-bonding [Os(NH,),I]’ I. By diazotisation with nitrous orbitals of the nitrogen molecule weakens the acid the mononitrogen complex yields the only N-N bond and so lowers the stretching known stable di-nitrogen complex (12): frequency, v(N=N). In nitrogen gas [OS(NH,)~(N~)]~++HNO,-+ v(N_N) is observed at 2331 cm-l in the c~~-[OS(NH,),(N,),]~++~H,O6 Raman spectrum. In the complexes it is The planar iridium complex trans- observed as a very strong sharp band in the [IrCl(N,)(PPh,),] was the second nitrogen infrared spectrum at 180-320 cm-l lower complex to be discovered. It is obtained very than in nitrogen gas. The frequencies have easily by the reaction of Vaska’s compound, been listed, together with the complexes in truns-[IrC1(CO)(PPh3),1,withacidazides(13): Table I. Because the band in the infrared CHC1,- spectrum is intense, the electrical asymmetry trans-[IrCl(CO)(PPh3),]+PhCON37 oc of the complexed nitrogen molecule is prob- trans-[IrCI(N,)(PPh,),] +PhCONCO 7 ably high. However, neither the electrical Platinum MetalsTev., 1969, 13, (1) 11 asymmetry nor the reduction in bond order complex is obtained, if at all, only as a very is sufficient to increase materially the re- unstable and impure material. Its presence activity of the nitrogen molecule in the com- can be established, from the N, stretching plex and there is no evidence that it can band in the infrared spectrum of the crude be either hydrolysed or reduced in aque- product, but it decomposes on attempted ous solution.